Bell and spigot joints are commonly used to join pipes, including extruded plastic pipes. Bell and spigot joints typically have three components; a bell on an end of a pipe, a spigot on an end of another pipe, and a gasket. These systems typically form a water tight joint.
Typical extruded multi-wall pipe includes a corrugated layer made using an extrusion process including corrugators. Bell and spigot joints are formed during the extrusion process using pipe corrugators incorporating pipe molds and a bell blocks. For example, see U.S. Pat. No. 5,405,569. The preferred process is to apply a heated gas or fluid between the outer shell and inner liner extrusion layers to form the bell and spigot.
There are two well known methods for forming a bell on the end of an extruded multi-wall corrugated pipe during the extrusion process. The first is a single extrusion layer bell, which is formed from the outer shell extrusion layer. Single layer bell extrusion processes often include complicated corrugators and extruder controls to help thin or thicken the bell, slowing down the pipe extrusion process.
The second method for forming a bell on the end of an extruded multi-wall corrugated pipe during the extrusion process results in a bell comprised of two plastic layers formed from the outer shell and an inner liner extrusion layer being fused together. In this process, the bell is formed by evacuating the air from between the two layers during the extrusion process. This process is complicated and is also known to slow down the extrusion speed of the corrugators.
Bell design involves several issues which have caused problems in the past. Control of the bell finish diameter is significant in the performance of a bell and spigot joint. For example, the bell must have adequate strength, through reinforcement or otherwise, to maintain a cylindrical shape during transportation and usage. The bell must be able to hold its shape during spigot and gasket insertion and subsequent pressurization of the pipe assembly.
One method used in the past to add strength to a pipe bell was to use reinforcing stiffeners, such as annular ribs molded into the bell. These stiffeners add strength and help maintain roundness, but typically create undulations in the inner surface of the bell. Undulations or irregularities have been known to cause problems of gasket rolling when a bell and spigot joint are assembled, as the gasket may be caught on the reinforcing ribs.
It is well known that plastic materials can have numerous variables affecting the shrinkage rates during processing. In both of the known methods of forming an inline bell discussed above, the sealing surface of the inner bell is subject to the shrinkage variability. This can cause significant dimensional control issues. For example, rapid cooling of the bell may create internal thermal stresses which may result in deformation. Differential deformation between the bell and spigot of the pipe joint may also result in leakage of a pipe joint.
Controlling the circumferential strain in the bell is important to prevent deformation of the bell during the pipe joining process. Controlling bell strain is also important for bells subjected to internal pressure. Bell expansion caused by sustained internal hydraulic pressure, for example, may result in loss of gasket seating pressure and of a water tight seal.
In the past, hose clamps and other external devices have been used to reinforce bell and spigot joints as a field fix for problem or leaking joints. It is desirable to eliminate the need for such external sealing aids.